JOURNAL OF RESEAR CH o f the Na tional Bureau of Standards - A. Physi cs and Chemistry

Vol. 72A, No.1, January- February 1968

Phase Relations Between Palladium Oxide and the Rare Earth Sesquioxides in Air

c. L. McDaniel and S. J. Schneider

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(August 22, 1967)

The eouilibrium prase relations were determined in an ai r environm ent between Pd~ and each uf th e foll owing: La, O", Nd,O", S m, O:" £u,O", Gd,O", Dy,O", Ho,O", Y,O", £r, O", Tm, O", Yb,O", and LU;03. In ai r Pd~ dissociates to Pd metal at 800 °C. The dissociation of Pd~ is apparentl y a reversible process. The Nd20 3·PdO and Sm,03·PdO sys tems were studied in de tail inasmuch as they typified se veral of the Ln,0 3·PdO sys tems. Three co mpounds , 2Nd20 3 . Pd~ , metas table Nd20 3 . Pd~ , Nd20 "

2PdO occur in .the Nd 20,,·PdO sys tem. The 2: 1, 1:1, ar.d 1:2 compounds, of unknown sy mmetry. dissociate or decompose at 1135, 860, and 1085 °C, respectively. The 2:1 compound di ssociates to the solid phases , Nd20 3 and' Pd. No furthe r reac tions occur between Nd20 3 and Pd up to 1300 0C. Three compound s, 2:1 , metas ta ble 1:1, a nd 1:2 occur in the Sm,0 3·PdO and Eu20 ,,· PdO sys tems. Two co mpound s, 2:1 and 1:2 occur in the La20:l·PdO sys tem. Other co mpounds de tec ted were the 1:1 and 1:2 in the Gd20 3·PdO sys tem and the metastable 1:1 in the DY20 3· PdO syste m. Each of th ese co mpounds subsequently dissoc iated upon hea ting. No apparent reaction occ urred between Pd~ and e ither H020 3, Y20", Er,0 3, Tm20 3, Yb20 3, or LU203.

Key Words: Di ssociation, equilibrium , Ln20 3: PdO co mpounds, Ln20 3·PdO sys tems, phase relations.

1. Introduction

Thi s s tudy is part of a program to de termine what effect , if any, various Pt-group metals have upon the metal ox ides when heated together in a n oxid izing e nvironm e nt. Several of the Pt-group me tals have a ctrong te ndency to oxidize wh en heated in air at mod­erate temperatures. At higher temperatures in air, these oxides volatilize and dissociate to one solid phase, the metaL Pre vious work [1 , 2] t has shown that iridium dioxid e reacts with other refrac tory oxides a t mod· erate tem peratures. Considering the fac t tha t several of the Pt-group metals are used as secondary standards on the International Practical Te mperature Scale (IPTS)2 [3] as well as container materials, it is im· portant to better understand the behavior of these metals in a n air e nvironment. This work presents the res ults of an inves tigation of the phase relations be· tween palladium oxide (PdO) and the rare earth sesquioxides (Ln2 0 3) in air.

The Nd 20:r PdO a nd S m203-PdO syste ms were studie d in de tail and were found to be quite similar in many respects . The prese nt study was broadened somewhat to include Pd~ in combination with each of the followin g sesquioxides: La203, EU203, Gd2 0 3,

I Figu res in brackets indicate th t: lit era ture refe~ence ~ a l I,he end of th is paper. :.t T his scale (lPTS) app lies to all temperatures listed In II1Is paper.

27

Dy20 :I, H020 3, Y20 :I , £r20 3, Tm20 :), Yb20 3 , and LUZO:I ' A limited s tud y see med adequ ate [or these te n sys te ms due to the ir simila rit y with the Ndz0 3-PdO sys tem or to their appare nt lac k of reac tion,

Palladium (Pd) oxidizes to Pd~ whe n heated in air at moderate te mpe ratures, Palladium oxide rather than Pd me tal was selected as one e nd me mber of the sys tem, becau se the latte r oxidizes too s lowly in air. By utilizing Pd~ , an a pp roac h to equilibrium could be achieved more readily. The study would still re fl ect , however , the be havior in air of Pd metal in combina· tion with other oxides.

Palladium has th e stru cture of face·centered c ubic copper with ([ = 3.8898 A [4), The freezing point of Pd is 1552 °C, a value which is given as a secondary reference point on the International Practi cal Tempera· ture Scale of 1948. The stru cture of Pd~ has been described on the basis of a tetragonal unit cell with ([ = 3.0434 A and c=5.337 A [5]. Upon heating, Pd~ has been re ported to di ssociate to Pd and a vapor phase at 870 °C in 1 atm oxygen [6].

The s table modification of neodymium sesquioxide (Ndz0 3) has the hexagonal A type (([ =3.831 A, c= 5.999 A) [7] rare earth oxide structure a t the tem­peratures investigated in thi s s tud y. Sa marium sesquioxide (Sm20 3) has been reported to c rys tallize in the C form at low temperatures and to invert di ·

rectly and irreversibly in air to the B type monoclinic structure at about 950°C [8]. The unit cell dimensions of B type SmZ0 3 were reported by Roth and Schneider [8] as a=14.16 A, b=3.621 X, c=8.84 A, and f3 = 100.05°. The melting points of Ndz0 3 and SmZ03 have been reported to be over 2000 °C [9].

2. Materials

All starting materials employed in this study had a purity of 99.7 percent or greater. With the exception of Pd~ and Y20 3 , the oxides were used in other inves­tigations and their spectrochemical analyses were reported previously [10, 11]. The Pd~ and YZ0 3

samples were found by general qualitative spectro­chemical analysis 3 to have the following impurities:

Pd~: 0.01-0.1%, Fe and Si;

0.001-0.01 % each AI, Ba, Ca, Cu, Mg, Pt, and Sr;

< 0.001% each Ag, Mn, and Pb

YZ0 3 : 0.01-0.1%, Ca;

0.001-0.01% each AI, Ho, Tm, and Yb;

0.0001-0.001% each Cu, Fe, Lu, and Mg.

3. Experimental Procedure

Specimens were prepared from 0.4 g batches of various combinations of Pd~ and the rare earth oxides. Calculated amounts of each oxide, corrected for ignition loss, were weighed to the nearest milli­gram. Each batch was thoroughly hand mixed, placed in fused silica tubes (sealed at one end) and fired in a mufHe furnace for a minimum of 18 hr at 770°C and at 780°C. Succeeding each heat treatment, the materials were thoroughly hand mixed and exam­ined by x-ray diffraction techniques.

Following the preliminary heat treatments, por­tions of each batch were placed in the open silica tubes and fired in a platinum alloy wire-wound quench furnace at various temperatures for different periods of time. The specimen was air quenched by quickly pulling the tube from the furnace. Equilibrium was assumed to have been achieved when the x-ray pat­tern showed no change after successive heat treat­ments or when the data were consistent with the results from a previous set of experiments.

Sealed platinum tubes were employed as speci­men containers for the experiments having pro­longed heat treatments below the dissociation tem­peratures. Sealed tubes were utilized in an attempt to maintain composition and to obtain maximum reaction. The use of fused silica tubes instead of platinum was necessary because Pd, frequently found as a decom-

3 The spectrochemical analyses were performed by the Spectrochemical Analysis Section of the National Bureau of Standards.

28

pOSItIOn product, readily reacts with platinum. On the other hand, the silica tube did not appear to in­fluence or react with the various oxide samples.

Temperatures in the quench furnace were measured with a 100 percent Pt versus 90 percent Pt-lO percent Rh thermocouple. All reported ·temperatures pertain­ing to quench furnace data are considered accurate to within ± 5°C. The precision of the measurements was ±2 0C.

All specimens were examined by x-ray diffraction at room temperature using a high angle recording Geiger counter diffractometer and Ni-filtered Cu radiation.

4. Results and Discussion

The equilibrium phase diagram for the Ndz0 3-PdO system in air is given in figure 1. The diagram was constructed from the data listed in table 1. The solid circles indicate the compositions and temperatures of the experiments conducted. It should be emphasized that figure 1 represents a composite of the Ndz0 3-Pd and Nd20 3-PdO systems in the Nd-Pd-Oxygen ternary. At the lower temperatures, the oxygen content of the specimens closely conforms to the compositions represented by the pseudobinary system, Nd20 3-PdO. At the higher temperatures, the compositions of the solid phases change by an apparent oxygen loss to those indicated by the Ndz0 3-Pd join. Figure 1 is a pseudo binary representation of a portion of the ter­nary system. This method of illustration has been employed by a number of investigators [1, 12].

Palladium oxide was found to dissociate to Pd metal and presumably oxygen at 800 ± 5 °C in air at atmospheric pressure. This value compares favorably with the data (870°C in one atmosphere oxygen) given by Bell et al. , in their study of the Pd-oxygen system [6]. The dissociation of Pd~ is a reversible process. Palladium oxide was first heated above 800 °C,- until only Pd was present. The same material was then reheated at 790°C and the x-ray data indicated only PdO.

Raub [13] reports that r,alladium takes up appreci­able quantities of oxygen into solid solution when the metal is heated in oxygen at 1200 °C. Raub's conclu­ions were based on weight gain data with no x-ray results given. Chaston [14] concluded the weight gain of Pd observed by Raub was due to the oxidation of base metal impurities. The x-ray · diffraction data obtained in the present study show no indication of solid solution of oxygen in Pd, when heated in an air environment.

Three intermediate compounds, 2:1, 1:1, and 1:2 occur in the Ndz0 3-PdO system. The stable 2:1 and 1:2 compounds dissociate to two solid phases and a vapor phase, presumably oxygen at 1135 and 1085 °C, respectively. The 1:1 phase was found to decompose at about 860°C to the 2: 1 and 1:2 compounds. In order

1400

1300 fo-

1200 fo-

fo-u

I 100 0

W 0::: ~ f-

1000 <:( 0::: W ll.. ~ w 900 f-

800 I-

I I

.. • • •

• NdZ 0 3 + 2 : I

•

I I

10 20

I I I I

• Ndz0 3 + Pd

• • •

1135° • ·

I 2: 1+ Pd • I

• • • • 2 :1 + 1:2

• 860 0

----------------t-------------• •

•

1 30

2: I

2:1 +f:/ •

I

40

• : . I

50 /:/

1:1 + /,'2 •

~

60

COMPOSITION, MOLE %

I

•

• • • •

• • •

• •

1

F, GU RE 1. Phase equilibrium diagram for the Nd,03·PdO system in air.

I I

-

1085° -

-

1:2 + Pd

Pd

800° ~

1:2+PdO

I

80 I

90 100

PdO

Dotted lint'!' indicait' Tllt:'tuf'la hl e I : 1 compou nd a nd dc('ompo:,iti(lll tem perat u re. It alic l('lterin~ indicalt-.<o lllf'ti.!!'lable phase assem blages.

e -composi tion !" and tf'llqwraturt-'s or f'x lH'rinwllt!' ('onciu(' I(-'d.

TABLE 1. Experimental data for compositions in the Ln,O:rPdO systems

Heat treatment" System Composition X·ray diffraction ana lyses b Remarks

Te mp. Time

mole % °C hr Nd,O,,· PdO ............ .. .. 75:25 780 18 2:1 + Nd20" + 1:1.. .... ......... ... Nonequilibrium.

c 1000 18 2:1 + Nd20 3

c 1063 3 2:1 + Nd20 3 c 1103 2 2:1 + Nd,O" ................. .... .. ... Quenched in ice water. d 1125 2 2:1 + Nd,03 c 1130 62 2:1 + Nd,O" .......................... Quenched in ice wat er. d 1130 1.5 2:1 + Nd20 3 d 1135 1.5 2:1 + Nd,03 + Pd ... .. .. .... ........ Nonequilibrium. d 1140 2 2:1 + Nd20 3 + Pd ......... .. ....... . Nonequilibrium.

71:29 780 22 2:1 + Nd20" + 1:1.. ............... . Nonequilibrium. c 1000 78 2:1 + Nd20 3

66.6:33.3 780 18 2:1 + Nd 20 3 + 1:1.. ................ Nonequilibrium. 790 18 2:1 + Nd,O" + 1:1.. .. .. ........ .... Nonequilibriulll . 800 20 2:1 + Nd20 3 + 1:1.. .... . ........... Nonequ ilibriulll. 900 71 2:1 + Nd,03 ... .. ........ ...... ....... Nonequilibrium e

c 1000 18 2:1 c 1050 504 2:1 c 1070 2 2:1 ..... . ........... ........ . .. ..... ... .. Que nched in ice water.

29

TABLE 1. ExperimentaL data for compositions in the Ln,0 3·PdO systems- Continued . Hea t trea tment a

System Co m posi tion X·ray diffrac tion analyses b Remarks Temp . Time

moLe % °C hr

c 1085 2 2: 1 ll15 20 2: 1 + Nd,0 3 + Pd ....... . .... .. ...... No nequilibrium ; reheat of 1205

°C specim en. dIllS 2 2:1 d 1125 2 2: 1 d 1130 2 2:1 d ll35 2 2:1 + Pd + Nd,O" ... .. . ........ . . .. .. Nonequilibrium.

1205 1 Nd,03 + Pd 1300 1 Nd20 3 + Pd

62:38 780 18 2:1 + 1:1 + Nd,O" ...... ..... ........ Nonequilibrium . 845 20 2:1 + 1:1

c 1000 78 2:1 + 1:2 50:50 780 18 1:1 + Nd,03 + PdO ........... ..... Nonequilibrium.

c830 20 1:1.. .......................... ... ....... Qu enched in ice water. 842 15 1:1 + Nd,03 .. ................ .. ..... Nonequilibrium ." 845 144 2:1 + 1:2 .............. ...... .. . ....... Reheat of 902 °C specimen. 855 3 1 :1 860 2 1:1 + 1:2 .. ...... .. ........ .... ........ Nonequilibrium. 865 2 1:1 + 1:2 + 2:1.. ..... ........ . ... .. . Nonequilibrium. 902 18 2:1 + 1:2

c 1000 18 2:1 + 1:2 c 1070 78 2:1 + 1:2 ................ ... .. . .. ...... Quenched in ice wate r.

1080 2 2:1 + 1:2 1085 2 2:1 + 1:2 + Pd ...... .. .. ... .......... Nonequilibrium.

1090 2 2:1+1 :2+Pd .. ....... .............. Nonequilibrium. 1100 4 2:1 + Pd + 1:2 ......... ... ........... Nonequilibrium. 1200 18 Nd,03 + Pd 1250 2 Nd,03 + Pd

38:62 780 21 1:2 + 1:1 + PdO ........ ..... .... .... Nonequilibrium. c845 20 1:2+1:1.. ................ ....... ..... Quenched in ice water.

c 1000 78 1:2+2:1 33.3:66.6 780 18 1:2 + 1:1 + PdO ....... ... .. ........ Nonequilibrium.

c800 120 1:2 900 2 1:2 925 2 1:2 970 120 1:2 + Nd20 3 + Pd .................. .. Nonequilibrium; reheat

1150 °C specimen . 975 2 1:2

c 1000 18 1:2 ..... . .... ........ .. ...... . ...... ...... Quenched in ice water. c 1000 48 1:2

1025 2 1:2 1040 2 1:2

c 1050 110 1:2 dl060 2 1:2 d 1070 3 1:2 d 1080 2 1:2 d 1085 2 1:2 + Pd +2:1... ... . . . .... . ..... ... Nonequilibrium. dl090 1.5 1:2 + Pd + 2:1.. ......... . ... ........ Nonequilibrium.

1110 18 2:1 + Pd 1130 2 2:1 + Pd ll50 1 Nd,0 3 + Pd

29:71 780 21 1:2 + 1:1 + PdO .... ...... . .. ........ Nonequilibrium. 1000 3 1:2 + Pd 1020 1.5 1:2 + Pd

25:75 780 18 1:2 + 1:1 + PdO .. ... ........... .. ... Nonequilibrium. 1000 3 1:2 + Pd 1050 1.5 1:2+ Pd 1075 2 1:2 + Pd 1085 3 Pd + 2:1 + 1:2 .. ... ... .. .... ... ...... Nonequilibrium. lllO 2 Pd + 2:1 ll50 2 Pd + Nd20 3

1300 1.5 Pd + Nd20 3

0:100 780 18 Pd~

Sec footnotes at end ur table . p. 33.

30

TABLE 1. Experimental data for compositions in the Ln203·PdO systems - Continued

Heat treatment a 5ystem Composition

Temp. Time

mole % °C Itr

795 2 800 2 810 4 825 1.5 850 3 900 3

1200 2

5m20~ PdO ..... . . .... . . . . . 75:25 780 18 " c 1103 3

1140 1 66.6:33.3 780 19

790 120

c 1000 20 c 1000 24 d 1100 2.5 d Ill0 2 d 1115 2 " 1125 1.5

1200 2 50:50 780 21

800 144 c830 20

830 5 850 2

880 2.5

c902 18 915 1.5

935 2

940 2

1000 20 1100 3 1200 2.5

33.3:66.6 780 21 790 120

c800 54 c 1000 20 d 1055 2.5 d 1060 2 d 1075 2

1150 1 25:75 780 21

1050 1.5

66.6:33.3 780 20 c 1000 18 c 1000 72 c 1070 66

1150 2 1160 3 1185 2 1190 2

50:50 780 20 850 144

c 1000 18 1000 144

1310 2

~(>f' fOO l nO ff' !' at I' nd of f<:Ible. 11 . 33. 31

X·ray diffraction analyses b Remarks

Pd~ Pd~ + Pd .......... .. .... .. .. .. ....... Nonequilibrium. Pd Pd Pd Pd Pd

2:1 + 5m20 3 + 1:1.. ......... ... .... Nonequilibrium. 2:1 + 5m,0 " ...... .. . .. . . .. .. .. ..... Quenched in ice wate r. 2:1 + 5 m20 ;] + Pd ...... . ..... .... .. Nonequilibrium. 2:1 + 5 m20 ;] + 1:1 .. .. .. ........... Nonequilibrium. 2:1 + 5 m20 " + Pd~ .. .. ...... . .. .. Noneq uilibrium ; reheat

1200 °C specimen. 2: 1 + 5 m,0 " ..... .. .... .. ....... ... Nonequilibrium e

2:1 2: 1 2: 1 2: 1 + Pd + 5 m,O". . .. .... .. .. .... No neq llili brium . 2: 1 + Pd + 5 m20 ;] .. .. . .. ......... No nequilibrium . 5 m,03 + Pd 1:1 + 1:2 + 5 m20" .... .... .. ..... .. Nonequilibrium . 2 :1 + 1:2 ..................... ... ...... Reheat of 1000 °C s pecim en. 1:1. ................. .. ...... .. ...... Quenched in ice wa te r. 1:1 1:1.. .... .. . .... .... ..... ........ Reheat of 830 °C 20 hI'

specim en. I :1.... ... ... . .... .. .. .. ...... .. ... .. ... Reheat of 830 °C 20 hI'

specimen. 1:1 .... ..... . ...... . .. .... . .. . ........ Quenched in ice wa te r. 1:1. .. .... . .. .. ............. . .. ... . .... Reheat of 830 °C 20 hI'

1:1. ..

1:1 + 1:2 +2 :1 .......

2 :1 + 1:2

specimen. Reheat of 830 °C 20 hr

specim en. Nonequilibrium ; rehea t of

830 °C 20 hr specimen.

2:1 + Pd + 1:2....... . ......... ..... Nonequilibrium . 5m20 " + Pd 1:2 + 1:1 + 5 m,O"........ .... .. .. . Nonequilibrium. 1:2 + Pd~ + 5 m,O;] ..... ... ....... . Noneqllilibrium ; reheat of

1150 °C specimen. 1:2 ... ..... ..... .... ...... .... .. ... .. .. Quenched in ice wate r. 1:2 1:2 1:2 + Pd......... .... .. .. .. . .. .... .... Noneqllilibrium. 1:2 + Pd + 5m,0 3... ... . . ..... ..... Nonequilibrium . 5m20 " + Pd 1:2 + Pd~ + 5m20"....... .. .. .. ... Nonequilibrium. 1:2 + Pd ... .. ........ .... ... .. ......... Quenched in ice water.

La,O" + Pd~ + 1:2 ...... .. ..... .. .. 2:1 + La20 " .. .. .. .. ...... ............ . 2:1 + La203 .............. ........... .. 2:1. ................... .. .. .... . . .. ..... . 2:1. .... ............... . ............. .. .. 2:1. .... . .. ... .... .. .... ........ . ....... . 2:1. .... .... ............ .... .. .. .. .. .. .. . 2:1 + La20 3 + Pd .. .. .. ........... ..

La20 " + Pd~ + 1:2 ...... .... ...... . 2:1 + 1:2 2:1 + 1:2 2:1 + 1:2 + Pd .. ........ .. .. ...... .

La20 3 + Pd + La(OH), ........... ,

Nonequilibrium. Nonequilibrium.e

Nonequilibrium.e

Quenched in ice water. Reheat of 1070 °C specime n. Reheat of 1070 °C specim en. Reheat of 1070 °C specime n. Nonequilibrium ; rehea t of

1070 °C spec imen. Nonequilibrium.

Noneq uilibrium ; reheat of 1310 °C specimen.

La20 3 co mmonly hydra tes at roo m temperatu re .

TABLE J. Experimental data for compositions in the Ln,O:l-PdO systems-Continued

Heat treatment a

System Co mposition X-ray diffrac tion analyses h Remarks Temp. Time

mole % °C hr

33.3:66.6 780 20 La203 + Pd~ + 1:2 .................. Nonequilibrium. e 1000 18 1:2 ..... . ...... ...... . ....... . ............ Quenched in ice water. e 1070 66 1:2 d 1085 2 1:2 d 1090 2 1:2 d 1100 2.5 1:2 d 1105 2 1:2+Pd .. ...... .. .. .. . ....... . .... .... Nonequilibrium. d 1110 2 1:2+Pd+2:1.. ........ ............. Nonequilibrium. d 1140 2 2:1 + Pd .......... ....... .. ... .. ... ... . Nonequilibrium.

Eu203·PdO ... .. . .. . ....... 66.6:33.3 780 18 EU203 + Pd~ + 1:1 + 2:1 ......... Nonequilibriu m. e900 10 2:1 + EU,03'" ...................... . Nonequilibrium! e950 48 2:1 + EU203 .......................... Nonequlibriume; quenched

in ice water. e 1050 36 2:1 + EU203 . ......... .. . .... .. .... ... Nonequilibriu m!

1085 2 2:1 + EU203 + Pd ... .. .. . ......... .. Nonequilibrium c ; reheat of 1050 o( specimen.

1090 2 2:1 + EU203 + Pd .. ....... ... .. ..... Nonequilibrium e; reheat of 1050 o( specimen.

1095 2 2:1 + EU20 3 + Pd ................... Nonequilibrium e; reheat of 1050 o( specimen.

50:50 780 18 EU203 + 1:1 + PdO .. ....... .. ...... Nonequilibrium. 800 144 1:2 + 2:1 + EU203 ................... Nonequilibrium; reheat of

1000 o( 20 hr specimen. e850 67 1:1 + EU203 .. . ... . . ..... .. ... ... ..... Nonequilibrium! e900 168 1:1 + EU203 ..... ......... . .. .. . ...... Nonequilibrium!

960 2 1:1 + Eu20" .................. ... .... . Nonequilibrium e; reheat of 900 o( specimen.

980 2 1:1 + EU203 .................. .. . ..... Nonequilibrium! 985 2 1:1 + EU203 + 1:2 .. ........ . .. . ..... Nonequilibrium. 990 2 1:1 + EU203 + 1:2 ................... Nonequilibrium.

1000 20 1:2 + 2:1 + EU203 .... ......... . . .... Nonequilibrium. 1000 144 1:2 + 2:1 + EU 203 ................... Nonequilibrium; reheat of

1250 °C specimen. 1250 2 Eu20" + Pd

33.3:66.6 780 18 1:2 + Pd~ + EU203 + 1:1 ......... Nonequilibrium. e900 10 1:2 + EU203 ... . ........... .. ..... .... Nonequilibrium e; quenched

in ice water. e950 96 1:2 1040 2 1:2 .......... ... ..... . ..... . ............. Reheat of 950 '( specimen. 1045 2 1:2+Pd +2:1.. ..................... Nonequilibrium; reheat of

950°C specimen. 1050 2 1;2 + Pd + 2:1.. ..... ....... .. . . .. .. . Nonequilibrium; reheat of

950 o( specimen. 1055 2 1:2 + Pd + 2:1.. ............ .... ... .. Nonequilibrium; reheat of

950 o( spec im en.

Gd20 ,,· PdO .. ... . . ... ... .. . 66.6:33.3 780 18 Gd20 3 + 1:1 + PdO ................. Nonequilibrium. e900 144 1:1 + Gd20 3

50:50 780 18 Gd20 3 + 1:1 + PdO .................. Nonequilibrium. e900 20 1:1 + Gd20 3 .......... ................. Nonequilibrium.c

900 72 1:2 + Gd20 3 + Pd ............. ....... Nonequilibrium; reheat of llOO o( s pecimen.

1000 2.5 1:1 + Gd20 3 ........................... Nonequilibrium c ; reheat of 900 o( 20 hr specimen.

1005 2 1:1 + Gd20 " + 1:2 ..... . .............. Nonequilibrium; reheat of 900 o( 20 hr specimen.

1010 3 1:1 + Gd20" + 1:2 ............. ....... Nonequilibrium; reheat of 900 o( 20 hr specimen.

1020 2 1:1 + Gd20 3 + 1:2 ................... Nonequilibrium; reheat of 900 o( 20 hr specimen.

1040 2 Gd20 3 + Pd + 1:2 .. .. ........... . ... Nonequilibrium; reheat of 900 o( 20 hr specimen.

llOO 3 Gd20 3 +Pd 33.3:66.6 780 18 1:1 + 1:2 + Gd20 3 .. . ......... . ...... Nonequilibrium.

e900 144 1:2 + Gd20" .. ....... ... ...... . .. . ... . Nonequilibrium.c

St'e footnutes at end of table. p. 33.

32

TABI.E 1. Expe rimental data for cOli/positions in th e Ln,O,,·PdO sys tems-Co ntinu ed

Heat Systl' m Compos iti on

Tcmp .

mole % °C

"950

1015

1020

1025

1030

1035

1040

Dy,O,,·pdO ......... . .. .. . 66.6:33.3 780 850

" 900 , 1000

50:50 780 ' 900

790

1000 1010 1025 1030

1250 33.3:66.6 780

'900 1050

50:50 780 , 790

800 900

Y,O,,·pdO ........ . . ... . . . 50:50 780 <790

900 1000

Er,O,,·pdO ..... ..... ..... . 50 :50 780 , 790 900

Tm,O,,· PdO 50:50 780 , 790

900 1000

Yb, O,,· pdO 50: 50 780 '790

900

Lu, O,,· PdO . . 50:50 780 '790

900 1000

I real m e nt a

Time

hI'

28

2

2

2

2

2

2

18 67 18

144 18 68

120

144 2 2 2

2 18 20

120

18 20

2 ]20

18 20

120 144

18 20

120

18 18

120 23

18 18

120

21 18 68 25

X·ra y diffrac tiun ana lyses" Remark s

1:2 + 1:1.. ....... . .. . . . ....... . . ....... Nonequilibrium c ; quenched

1:2 + Cd20 " ....... .. .. . ..... ..... ..... .

1:2 + Cd,O" ... .. ... ....... .. . ........ .

1:2 + Cd,O" + Pd . ... ' .. .... ........ .

1:2 + Cd20" + Pd ... .... ............ .

1:2 + Cd,O" + Pd. , ......... ........ .

1:2 + I'd + Cd,O" ... ... .. .. . .. .. .... .

1:1 + Pd~ + Oy,O" . .... . . . .. . .. .. . 1:1 + Oy,O" 1:] + Oy,O" .......... . .. . . .. .... . 0.

1:] + Oy,O" 1:1 + Oy,O" + pdO .... ... .. ...... .. 1:1 + Oy,O" .... . ................... . Oy,O" + Pd~ + I'd .. ... . ... .... .... .

in ice water. Nonequilibrium c ; reheat of

950°C specimen. Nonequi librium e ; rehea t of

950°C speci men. Nonequilibrium: reheat of

950°C specimen. Nonequilibrium; reheat of

950°C specimen. Nonequi li brium; rehea t of

950 °C speci men. Nonequilibrium; reheat of

950 °C specim en .

Nonequi li brium.

Quenched in ice water.

Nonequilibrium. Nonequilibrium.c Nonequ ilibrium; reheat of

1250 °C specim en. Oy,O" + Pd ............ .. .. . . . ........ Reheat of 1250 °C spec im en. 1: 1 + Oy,O" .. .. .... . ... . .......... . .. Reheat of 900 °C spec imen. 1:1 + Oy,O" .. . ..... .. . ... . . . . .... .... Reheat of 900 °C specinlt"n. 1: 1 + Oy,O" + Pd ................ . .. Nonequi librium; reheat of

Oy,O" + I'd l :l + PdO + Dy,O" .......... .. . . . l : ] + I'd I'd + Dy, O:,

Ho,O,, + Pd~ Ho,O,, + Pd~ Ho,O" + Pd~ + Pd ......... .. Ho,O,, + I'd

Y, O,, + pdO Y,O,, + PdO Y,O,,+ I'd Y,O,, + Pd

Er,O,,+ Pd~ Er,O" + Pd~ Er,O,, + Pd

Tm,O,, + Pd~ Tm,O" + Pd~ Tm, O" + Pd Tm,O" + Pd

Yb, O" + Pd~ Yb,O,,+ Pd~ Yb,O" + Pd

Lu,O,, + Pd~ Lu,O" + Pd~ Lu,O" + I'd Lu,O" + I'd

900 °C specimen.

Nonequi lih rium.

Noneq uilibrium .

"A ll specim ens were heat trea ted at 770 °C, a minimum of 18 hI'. Unl ess othe rwise indicated, fused s ili ca tubes (sea led at one end) were used for spec imen cont ainers and were a ir qu enched.

"The phases id entified a re given in orde r of the rela ti ve amou nt present at room temperature. C Sealed platin um tubes were used for speci men containe rs. d Rehea t of 1000 °C s pecime n. c' PdO proba bl y lost by vo latilization.

28 1- t97 0-68-3 33

to establish the stability of the three compounds , appropriate mixtures were heated first above and then below their respective decomposition or dissociation temperatures. Only the 2:1 and 1:2 compounds re­formed from their decomposition products. Prolonged heat treatments (up to 6 days) failed to reform the 1:1 compound, indicating perhaps it is metastable phase that forms only on heating.

A literature search did not reveal any compounds structurally similar to the Nd20 3-PdO phases. The x-ray powder patterns of the compounds were not similar to those reported by Barry and Roy [15] for the 2:1, 1:1, and 1:2 rare earth oxide· calcium oxide compounds. The unindexed x-ray diffraction powdpr patterns for the three compounds found in the present study are given in table 2.

At temperatures above 800 °C the syste m no longer can be represented by the Nd20 3-PdO join. The sys­tem changes through dissociation and at 1135 °C becomes the true binary Nd2Q3-Pd. Up to 1300 °C , Nd20 3 and Pd do not react in the solid state.

TABLE 2. X-ray diffraction powder data f or Nd20 3·PdO compounds

«(u K. radia tion)3

2Nd,o, PdO' Nd20 3 • PdC Nd,O, . 2PdO'

d III, d 11/0 d

3.2840 15 3. 1895 17 4.2387 3.0064 100 2.8768 100 3.4568 2.8723 45 2.8280 60 3.2733 2.8643 81 2.0732 12 2.9303 2.8038 45 1.9980 33 2.8228

2.1401 21 1.9556 8 2.7792 2.0827 20 1.67 15 18 2.5924 2.0170 8 1.6423 27 2.3178 1.9746 19 1.4393 7 2.1372 1.9313 12 1.4120 9 2.1185

1.7640 8 1.2641 5 2.0732 1.7199 9 2.0347 1.6888 12 2.0179 1.6800 22 1.8192 1.6628 4 1.7804

1.61 37 11 1.728 7 1.6019 II 1.6634 1.5784 10 1.6148 1.4342 6 1.S898

1.5732

1.5674 1.5230 1.4669 1.4181 1.3913

1.3807 1.2971 1.2753 1.2281 1.1870

ad _ inte rplanar spacing, 1110 - relative intensit y. b X-ray pattern obt.ained from speci men heat trea ted at 1000 °C for 18 h r. e X_ray pa lte rn obtained from s pecim en heat treated a l 830 °C for 20 hr.

Ilk

13 12 20 28 76

41 100 7 1 12 10

6 9

54 20 8

II 7

18 II 25

33 21 15 7

II

3 II 8 4 7

-

The equilibrium ph ase diagram for the Sm20 3-PdO sys te m in air is given in fi gure 2. The pertinent data are listed in table 1. The diagram is s imilar to the Nd2 0 3-PdO system in many res pects in that three intermediate compounds 2:1, metastable 1:1 , and 1:2 also occur. These phases dissociate or decompose at 1115 , 940, and 1060 °C , respectively.

L 34

4.2. Other Ln20 3-PdO Systems in Air

Mixtures were prepared from Pd~ and eac h of the following sesquioxides: La20 3, EU20 3, Gd20 3, Dy20 3, H020 3, Y 20 3 , Er20 3, Tm 20 3, Yb20 3, and LU20 3. The phase equilibrium diagra ms for the various Ln20 3-Pd~ s ys tems in air are given in figure 3. The experi­mental data are tabulated in table 1. It is ev.id ent that the Nd20 3-PdO sys tem is representative in a general way of the other Ln20 3-PdO systems. T he La20 3-PdO diagram indicates the occurrence of the 2: 1 and the 1:2 compounds. The 1: 1 compound was not detected. Three compounds, 2:1 , metastable 1:1, and 1:2 occur in the Eu20 3-PdO system. The 1:2 and metastable 1:1 compounds occur in the Gd20 3-PdO system. The 2Gd20 3 . Pd~ compound was not detected. In the DY2 0 3-P dO sys tem, only the metastab'le 1:1 compound was detec ted.

The remaining syste ms of either H020 3, Y20 3, Er20 3, Tm2 0 3, Yb20 3, or LU20 3 with Pd~ are rather simple and straightforward inasmuch as there was no detect­able reac tion between end me mbers in t he solid state. The diagrams indicate only the dissociation of Pd~ , the point at whic h the system reverts to the true Ln20 3-Pd sys te m.

Ta ble 3 summarizes the results obtained in this study. Listed are the systems inves tigated , size of the rare earth cation, and the di ssociation temperatures of the Ln20 3-PdO co mpounds. The dissociation temperatures of the 2:1 compounds decrease as the size of the rare earth cation decreases. The 1:2 com­pounds were found to di ssociate in a similar manner. However , the decomposition te mpera tures of the metastable 1:1 compounds increase as the size of the rare earth cation decreases . As expected, the Ln20 3-P dO compounds with the same molecular ratio appear to have similar x-ray patterns. The patterns indicate appropriate shift in d-spacings to account for difference in ionic size of the r are earth cation s.

It should be noted that the proposed diagra ms per­tain only to the phase rela tions of the sys tems in an air environment at atmospheric pressure . Any change in oxygen pressure would chan ge th e equilibrium diagram. In an air environment, precaution should be taken when utilizing Pd metal in combination with some of the rare earth oxides , since the data indicate the tendency to form new phases.

4.3. Summary

Equilibrium phase diagra ms for systems involving Pd~ and various rare earth oxides were determined in air. Selected mixtures ,in the systems were studied by x-ray diffraction techniques after various heat treatments. Palladium, in air , oxidizes to Pd~ at moderate temperatures. The dissociation te mperature of Pd~ in air at atmospheric pressure was established at 800 ± 5 0c. This dissociation was fo und to be a reversible process. P alladium oxide r eacts with a number of oxides to form binary compounds. The pseudobinary sys tem Nd20 3-P dO exemplified the typical type of reaction and was studied in detail. Three compounds, 2Nd20 3· Pd~ , Nd20 3· Pd~ (meta-

1400 I I

1300 -

1200 -I u > 0 • -

W 0:: 1100 l- • ~ I-<I: 0:: W 1000

> 0... ~ LLl

l-

Sm203 + 2 :1

I-

900 l-

~ 800 l-•

700 I I

10 20

I I I I I

Sm203 + pd

• •

• • 1115 0

• 2:1 + pd •

2: 1+ 1:2 •

940 0 ......... .... ....... ... ; .... .... ...... .. ...... .. ,

• • 2 :1+1:1 • 1·"/ + 1:2 •

t •

I I I I

30 40 50 1:1

60 70 2:1 1:2

•

•

I I

-

-

-10600

-

1:2 + pd

-

pd 8000 ~

1:2 +

I

80

pdo I

90 100 pdo

> COMPOSITION, MOLE %

.. ,

F IGU RE 2. Phase equilibrium diagram/o r the Srn,O,,· PdO system in ai r. Dotted lines in~ica l e mctasta hle 1:1 compou nd and decomposition temperature. It a lic IClle ring ind icates me tastab le phase asse rnblctgcs .

• -compos itions and tcmpenl lures of experim e nt s cond ucted.

s ta ble), and Nd20 3·2PdO occur in the sys te m. The 2: 1, 1:1 , and 1:2 compounds, of unknown symme try di ssociate or decompose at 1135, 860, and 1085 °C, respectively. Above 1135 °C the sys tem corres ponds to the Nd20 3-Pd binary system. No further reac tion appears to take place between Nd20 3 and Pd up to 1300 °C. Similar type compounds were found to exis t in other Ln20 3-PdO syste ms.

Three compounds, 2:1, 1:1, and 1:2, occur in the Sm~O:J-PdO and Eu20 :J-PdO systems. Two compounds,

35

2:] and 1:2, occur in the La20 3-PdO sys tem. Other co mpounds detected were the 1:1 and 1:2 in the Gd20:J-PdO sys te m and the 1:1 in the Dy20 :J-PdO syste m. Each of these compounds al so di ssociated upon heating at temperatures above the dissociation te mperature of PdO. Mixtures of either HoZ0 3, Y20 3, Er20 :J, Tm20 3, Yb20 3 , or LU20 3 with Pd~ did not react in the solid s tate.

1400 r -

1190 0

2 : I+-pd- - -@5~= - - - - -' -""---,--

1000 - 1:2+ pd

2 : I + I: 2 800 0 p~ 1:2 + pdo

600 1 1 I I

L020 3 2 : I 1:2 pdo

1400 - -

EU203 + pd

1085 0 2:1 + pd

-- --- -/-- - -IO~~-=-1----1000 -~ ... :?'cS'5..o . 1:2 + pd

2:1 + 1:2-2 :1 ~ 1:1 8000 P-'l.

EU203 + 2:1 + + 1:1 1."2 1:2 + pdO

I I : I I 600 EU203 2 : I I: I I: 2 pdo

- -

-pd~

800 0 ---..........

Y20 3 + pdo 1 1 I 1

,- -

Er203 + pd

-pd

800"

-----------Er20 3 + pdO I I I I

Er20 3 pdo

U 1400- - - -o

-W 0: ::J f­<I: 0: W Q

2 w f-

1000

1000

600

Gd 203 + pd Gd203 + 1:2

______ ~ 10.f~0 __ =··········)005';·· , ........ 1:2 + pd

I I

DY20 3

1:1 8000 Pd ....... +

1:2 1:2 + pdO I I

/:I 1:2 pdo

1:/ pdo

-

Tm203

I

-

Yb203

Tm 20 3 + pd

pd 800 0 -~

Tm203 + pdo I 1 I 1

pdo -

Yb 203 + pd

800 0 pd -----.......

I I

pdo

1400 -

l - -

H0 20 3 + pd

1000 - -

800 0 Pd~

800 0 Pd~

H020 3 + pdo LU203 + pdo 1 I 1 1 1 1 1 1

20 40 60 80 100 0 20 40 60 80 100 pdo LU203 pdo

COM POSITION, MOLE %

FI GU RE 3. Proposed phase equilibrium diagrams for various Ln,O,,-PdO systems in air. For cla rity experinw ll ta\ points a re not inc luded. sec ta ble I fu r exac t compos itions and temperatures s tu died. DOll ed lines ind icate metas tabl e 1:1 compou nd a nd decumpositiun lem-

pt' rat ure. Italic lett e ring indica tes me tas table pbase assemblages.

36

'-

I ~

"I

J

J I

. ~

'1

>

I ~

RADIUS OF OISSOCIATION TE MP.oC

SYSTEM Ln+J A 2 Ln10J PdO Ln10J' PdO Ln10J 2 Pd ° La10J-PdO I . I 4 1190 - 1105

Nd1 OJ - Pd ° I .04 11 35 (8 60) 1085

Sm10J - Pd ° I .00 1115 (9 40) 106O

EU10J -PdO 0.98 108 5 (985) 1045

Gd10J -PdO 0.97 - (1005) 1025

Dy 1 OJ - Pd ° ° 92 - (10 3O) -

Ha10J-PdO 0.91 - -- -

Y10J -PdO 0.91 - -- -Er1 OJ - PdO ° 89 - -- -

T m10J - Pd ° 0.87 - -- -Yb 1 03 -PdO 0.86 - -- -LU10J- PdO 0.85 - - -

T ABLE 3. Dissociat ion lem peratures of Ln10 " . Pd~ compounds . ,\11 rad ii of the rare ca rth ca t ion s t a ken from Arhe ns [16 1 wi th the exception of Y ":L

which was ta ken fro m Roth a nd S(' hne ider 18 1. Pa re n thes is indica te d ccomposi tion l em pera­ture s of mc tas table I : I compounds.

5. References

[IJ S. J. Schneider, J. L. Waring. and R. E. Tress ler, J. Res. NB 69A Whys. and Chern .), No.3, 245- 54 (1965).

[21 c. L. Mc Daniel and S. J . Schn eider, J. Res. NBS 71A (phys. and Chern .), No.2, 119-23 (1967).

[31 H. F. Stimson , J. Res. NBS 65A Whys. and Chern .), No.3, 139-45 (1960).

[41 H. E. Swanson and E. Tatge, NBS Circ. 539, I , 21 (1953). [51 H. E. Swanson, R. K. Fuyat , and G. M. Ugrinic, NBS Circ.

539, 4, 27 (1963). [61 W. E. Bell , R. E. In ya rd , and M. Tagami, J. Phys. Chern. 70

[111 ,3735-36 (1966). [7J H. E. Swanson, R. K. Fuyat, and G. M. Ugrinic, NBS Circ.

539, 4, 26 (1963). [81 R. S. Roth and S. J. Schneider, J. Res. NBS 6 4 A (phys. and

Chern .) No. 4, 309-16 (1960). [91 S . J. Sc hneider, NBS Monograph 68. 31 p. (1963).

[101 S. J. Schneider and R. S. Roth, J. Res. NBS 64A (phys. and Che rn .) No.4, 317-32 (1960).

[ll J S. J. Schneider, J. Res . NBS 65A Whys. and Che rn .), No.5, 429- 34 (1961).

[1 21 A. Muan, Am. J. Sci. 256 , 171- 207 (1958). [l31 E. Raub, .J. Less Common Metals 1,3- 18 (1959). [14] .I . C. Chaston, Pl at inum Metals Rev. 9 [4], 126- 9 (1965). [151 T. L. Ba rry, V. S. Stu bican, and R. Roy, .J. Am. Cera m. Soc.

49 [1 2J, 667- 70 (l966). [161 L. H. Ahrens. Geochim. et Cosmochim. Acta 2 , 155-69 (1952).

(Pa per 72Al -479)

37